We study physical properties of a fossil shell associated with fading radio lobes in NGC~1275 (also known as 3C84). In our recent work of Ito et al., we have modeled the dynamical and spectral evolution of fossil shells which are identical to the forward shocks propagating in the external medium and found that the shell emission overwhelms that of the radio lobes soon after stopping the jet energy injection because fresh electrons are continuously supplied into the shell via the forward shock while the radio lobes rapidly fade out without jet energy injection.

By applying the model to NGC~1275, we find that, (i) shocked the thick torus can be detectable by VLBI several ten years after, and (ii) shocked ambient matter can be compatible to the sensitivity of the Cherenkov Telescope Array (CTA).